Dental implants are used to replace individual teeth or for anchoring more complex structures, which generally replace several or even all of the teeth.
Implants are often constructed in two parts, in which case they consist of an anchoring part, often referred to in isolation as the implant, and of a separate abutment. The anchoring part is either embedded completely in the bone, that is to say to the height of the alveolar crest, or protrudes by a few millimeters from the alveolar crest into the soft tissue. The abutment is mounted on the anchoring part and extends into the oral cavity to form a support for a dental prosthesis or denture.
During the lifetime of the prosthesis, which can be over 20 years, the implant system will be subjected to large loads caused by mastication. The abutment must therefore be firmly fastened to the implant in order to prevent loosening and potential loss of the component. This can be achieved in numerous ways, for example, via compression fit or gluing. However, screw fit connections are generally preferred. By applying a sufficiently high torque during attachment a firm connection between the implant and abutment can be achieved.
In many systems therefore the implant comprises an internal threaded bore, while the abutment comprises a corresponding apical thread, thus allowing the abutment to be screwed directly into the implant.
However, this has the disadvantage that the exact angular position of the abutment relative to the implant is not known until final fixation. This can have disadvantages, particularly when the abutment is intended to support a single tooth prosthesis.
Therefore, many implant systems comprise anti-rotation means, which prevent relative rotation between the implant and abutment and which set a finite number of angular positions which the abutment can have relative to the implant.
These anti-rotation means consist of complementary non-circular symmetric portions in the implant and abutment, usually having a polygonal shape such as a hexagon or octagon.
Such systems ensure that the exact angular position of the abutment in relation to the implant is known prior to fixation and can help to prevent loosening of the abutment during the lifetime of the implant.
Of course, when such anti-rotation means are employed it is not possible to rotate the abutment relative to the implant and hence the abutment can no longer be directly screwed into the implant. Therefore a third component, often a screw known as a “basal screw”, is used to connect the abutment to the implant.
When a basal screw is used the abutment typically comprises a screw channel extending through the abutment and having a screw seat. This enables the basal screw to be fed through the abutment until the screw head abuts the screw seat and for a screwdriver to be inserted into the channel to engage the screw and fasten this to the internal threaded bore of the implant, thus clamping the abutment securely to the implant.
An example of such a known implant system can be found for instance in EP1679049, in which the screw seat is conical and WO2006/012273, in which the screw seat is planar.
As well as abutments, basal screws are also used to connect other, temporary secondary components to the implant, for example, healing caps, closure screws and impression posts.
As mentioned above, it is important that the abutment in particular is firmly fastened to the implant in order to prevent loosening over the lifetime of the implant system. In the case of screw fit systems, this is achieved by tightening the screw component, whether this is a third component or the abutment itself, in order to achieve a high pre-load, or clamping force.
In order to achieve the maximum pre-load, it is desirable to tighten the screw as much as possible without reaching the yield strength of the screw. At this point, the tension within the screw body can result in plastic deformation of the threads and in some cases fracturing of the screw. This is highly undesirable as the screw must then be removed and replaced. Removal of a damaged screw is not always easy and furthermore this can result in damage to the internal threads of the implant. In some cases, the damage to the screw may not become apparent until after the final prosthesis has been fixed to the abutment, and hence the replacement of the screw can also result in the need for the creation of a new prosthesis.
Manufacturers of dental implant systems therefore set recommended maximum torque values, which ensure a high pre-loading of the screw without risking over tensioning.
However, given the natural desire to ensure a high pre-loading of the implant system, dental practitioners often apply a fastening torque significantly over the recommended value, which can lead to failure of the screw.
In order to increase the tensional strength of screws to prevent breakage in such situations, one potential solution would be to manufacture the screws from a different, stronger material. However, given the long term use of the screw within the human body any new material must undergo rigorous safety tests, and finding a new material having the necessary high strength together with the required biocompatibility is not a simple matter.
Another option would be to increase the dimensions of the screw. However, in dental implant systems space is restricted as the implant must fit within the available space within the jaw bone whilst removing as little bone mass as possible, to limit trauma at the implant site. Therefore the overall dimensions of the implant system can not be altered, and so any increase in the diameter of the screw would result in an equivalent reduction in the thickness of the implant and/or abutment. Such a modification would simply weaken the system in another area.